Heart disease is still one of the most prevalent medical ailments in the world. Intraluminal endovascular grafting, a type of angioplasty procedure, has been demonstrated by experimentation to present a possible alternative to conventional vascular surgery and is used to treat heart disease. Intraluminal endovascular grafting involves a tubular prosthetic graft or stent and delivery within the vascular system. As defined herein, the terms “graft” and “stent” are used interchangeably. Advantages of this method over conventional vascular surgery include obviating the need for surgically exposing, incising, removing, replacing, or bypassing the defective blood vessel. Over 20 million angioplasty or related procedures involving occluded vasculature have been preformed worldwide. About 30% of these angioplasties fail within 30 days. These failures typically require the procedure to be repeated.
Several years ago, a product called a stent, named after Charles Stent, was introduced for use in angioplasty procedures. The stent reduced the angioplasty failure rate to about 15 percent. A stent is an expandable metal tubular device that is mounted over an angioplasty balloon and deployed at the site of coronary narrowing. The balloon is inflated to expand the stent to physically open and return patency to the body passageway. After the stent is expanded, the balloon is deflated and removed and the stent is permanently disposed to retain the opened body passageway. The first generation of expandable stents did not offer a controllable radial expansion. An improved stent disclosed in U.S. Pat. No. 4,733,665 overcame the problem associated with controlled stent expansion. However, prior art stents still do not provide control over the final, expanded configuration of the stent. For instance, the expansion of a particular coiled, spring-type stent is predetermined by the method of manufacturing, material and delivery system. In the case of self-expanding intraluminal stents, or prostheses, formed of a heat sensitive material which expands upon exposure to core body temperature, the amount of expansion is predetermined by the heat expansion properties of the particular alloy utilized in the manufacture of the intraluminal stent. Consequently, once the foregoing types of intraluminal stents were expanded at the desired location within a body passageway, the expanded size of the stent could not be increased. If the proposed expanded diameter of the narrow body passageway was not determined correctly, the stent might not expand enough to contact the interior surface of the body passageway so as to be secured thereto and/or not expand the body passageway to the desired diameter. The stent disclosed in the '665 patent overcame the problems associated with these past stent designs.
The stent based upon the '665 patent is currently being used in angioplasty procedures. Stents, including the stent of the '665 patent, are presently used in approximately 30-60 percent of all angioplasty procedures. However, these stents have several shortcomings which contribute to procedural failure rates. The currently used stents are not readily visible under fluoroscopic guidance procedures. Stent placement is hindered as a result of poor visibility. As a result, precise positioning of the stent during the insertion procedure was difficult to achieve. Consequently, the stent could be inadvertently positioned in the wrong or non-optimal location in the body passageway. These stents also shorten longitudinally after radial expansion, which is not desirable for their intended use. The shortening of the stent resulted in longitudinal movement of the stent during expansion, which sometimes resulted in the stent being fully expanded in the wrong or non-optional position. One stent design was proposed in U.S. Pat. No. 5,853,419. The stent included a hexagon in the side wall of the stent which theoretically resulted in the stent retaining its longitudinal length during expansion. The stent also included ends that flared outwardly. However, in practice, the stent does not expand as described in the '419 patent. Due to the hexagonal configuration of the openings in the stent, the struts that form the hexagonal configuration cause the ribs of the hexagonal configuration to bend, buckle or twist when the struts are being expanded, thus resulting in a reduction in the longitudinal length of the stent. The bending, buckling or twisting of the ribs can only be avoided if the struts are made of a very flexible or bendable material; however, the use of such material compromises the strength of the stent. Not only does the stent not retain its longitudinal length, the complex stent design is both difficult to manufacture and uniformly expand in a body passageway.
The improved stent disclosed in U.S. patent application Ser. No. 09/273,736 filed Mar. 22, 1999, which is incorporated herein by reference, overcomes these past problems with stents. The patent application discloses an improved stent that can be coated with one or more substances in various regions of the stent to improve the visibility of the stent by various techniques (e.g. fluoroscopy) during the insertion procedure, thereby improving the positional accuracy of the stent in the body passageway. The improved stent also incorporates a unique design which enables the stent to retain its original longitudinal length during expansion. The improved stent also is easier to manufacture and substantially uniformly expands in the body passageway.
Although the improved stent overcomes the deficiencies of prior art stents with respect to accurate stent positioning, problems can still exist with respect to tissue damage by the stent during insertion and/or expansion of the stent. The two ends of prior art stents typically include one or more rough, sharp and/or pointed surfaces. These surfaces can cause irritation and/or damage to surrounding tissue as the stent is moved within the body passageways. Such irritation or damage to the surrounding tissue can create various types of complications during the surgical procedure. These surfaces can also cause damage to surrounding tissue during the expansion of the stent. During stent expansion, the middle of the stent is first expanded by the angioplasty balloon. As the middle of the stent expands, the ends of the stent move toward one another. This movement of the ends can result in the stent ends digging into and/or penetrating the surrounding tissue. Furthermore, tissue damage can result when the end portions of the stent are eventually expanded by the angioplasty balloon. Stent designs that have flared out ends can also cause damage to tissue during insertion of the stent and expansion of the stent. U.S. patent application Ser. No. 09/771,073 filed Jan. 29, 2001, which is incorporated herein by reference, includes a stent design that overcomes or minimizes tissue damage by the stent during stent insertion and stent expansion. The stent includes rounded and/or smooth edges for the end portions of the stent.
Several problems can develop after the stent is inserted into a body passageway. One problem is known as in-stent restenosis wherein the body passageway, which has been previously treated with a stent, renarrows or closes within the stented segment. The renarrowing or closure of the body passageway can be caused by a structural failure of the stent due to contractive forces by the body passageway on the stent and/or by the body passageway growing into the openings in the stent. Other problems can include vascular narrowing and restenosis. Vascular narrowing is defined as a vascular segment that has not been previously treated by any interventional means and eventually closes, thereby preventing fluid body passageway. Restenosis is the renarrowing of a previously treated vascular segment not involving a stent. Both of these problems are the result of a body passageway that was not treated with an invasive angioplasty, narrowing or closing, and from the insertion of a stent in one portion of the body passageway causing vascular narrowing or restenosis in another part of the body passageway. Vascular narrowing, restenosis and in-stent restenosis are caused by biological factors causing the premature closing of the body passageways. One such biological factor is platelet derived growth factor, referred to as PDGF. PDGF is an intercellular messenger capable of stimulating proliferation of smooth muscle cells. Smooth muscle cells are known to migrate within body passageways such as arteries and cause a restenotic reaction.
The problems with vascular narrowing, restenosis and in-stent restenosis are significantly overcome by the use of one or more drugs. U.S. Pat. No. 6,206,916 entitled “Coated Intraluminal Graft,” which is incorporated herein by reference, discloses the use of a drug coated on at least a portion of the stent to inhibit or prevent the occurrence of in-stent restenosis, vascular narrowing and/or restenosis. Although the intravenous use of drugs and/or the coating of the stent with drugs can inhibit or prevent the occurrence of in-stent restenosis, vascular narrowing and/or restenosis, the continued need for the drugs after the stent has been inserted can require the patient to be retained in the hospital for extended periods of time. Alternatively, in-stent restenosis, vascular narrowing and/or restenosis may occur days or weeks after the stent insertion procedure and after intravenous use of drugs has terminated and/or the drug coating on the stent has been dissolved off the stent. Several other United States patents disclose the use of various drugs coated on stents. For example, U.S. Pat. No. 5,716,981, which is incorporated herein by reference, discloses the use of paclitaxel or an analog or derivative thereof for use on a stent. U.S. Pat. Nos. 5,733,925 and 5,981,568, which are incorporated herein by reference, disclose the use of taxol or a water soluble taxol derivative; cytochalasin or analog thereof; or other type of cytoskeletal inhibitor for use on a stent. Several United States patents also disclose the use of polymers to bind the various drugs to the surface of the stent. Several of these polymers are disclosed in U.S. Pat. Nos. 5,578,075 and 5,679,400, which are incorporated herein by reference. U.S. Pat. No. 5,464,650, which is incorporated herein by reference, discloses the method of applying several coatings of a polymer that has been mixed with a drug so as to control the delivery of the drug in a body over a period of time. The method of coating the stent involves a series of steps that significant increases the cost, complexity and time for the manufacture of the stent.
In view of the present stent technology, there is a need and demand for a stent that has improved procedural success rates, has higher viability under fluoroscopy in vivo, retains its longitudinal dimensions from its original pre-expanded configuration to its expanded configuration, minimizes damage to tissue during insertion and expansion of the stent, inhibits or prevents the occurrence of in-stent restenosis, vascular narrowing and/or restenosis long after the stent has been inserted into a body passageway, and is simple and cost effective to manufacture.